The present application generally relates to metal or metallic articles such as jewelry that are coated or plated with metal or metallic coatings.
Plating is a process where a thin layer of metal is deposited on the surface of a substrate. Metals are plated for various reasons, including for decoration, to harden, to alter conductivity, to inhibit corrosion, to reduce friction, to improve paint adhesion, to improve solderability, to improve wearability, and for radiation shielding. Gemstones can also be plated to provide improved color characteristics. See, e.g., U.S. Pat. No. 5,853,826.
Processes used in plating include electroplating, physical vapor deposition (PVD) and chemical vapor deposition (CVD). In electroplating, an electrical current is used to reduce cations of a coating material from a solution to coat a conductive substrate with a thin layer of the material. In PVD, a vaporized form of the coating metal is condensed in a vacuum onto the substrate surface. Vaporization and deposition of the coating metal can be effected by a number of methods known in the art, including evaporative deposition, electron beam physical vapor deposition, sputter deposition, cathodic arc deposition, pulsed laser deposition, and plasma-spray deposition. CVD involves exposing the substrate to a volatile precursor of the coating metal, which reacts or decomposes on the substrate surface.
While electroplating is the simplest process of plating a metal, electroplating is difficult or impossible where the substrate is a refractory metal such as tungsten, molybdenum, niobium, tantalum or rhenium. In particular, tungsten and tungsten alloys such as tungsten carbide cannot be electroplated. Thus, a tungsten or tungsten alloy article such as an article of jewelry cannot be directly electroplated with, e.g., a layer of a precious metal such as gold, platinum or rhodium. This makes production of plated articles of a tungsten or tungsten alloy substrate difficult to produce. There is thus a need for new methods of making such articles. The present invention addresses that need.